1. Field of the Invention
The present invention relates to a process for producing phenyl phosphonothioic dichloride (hereinafter referred to as PPTC). More particularly, it relates to a process for producing PPTC by reacting benzene with thiophosphoryl trichloride in the presence of a catalyst under special conditions.
2. Description of the Prior Art
Phenylphosphonothioic dichloride (PPTC) is a common intermediate for the production of insecticides and flame-proofing agents. It has been disclosed to produce PPTC by reacting benzene with phosphorus trichloride in the presence of aluminum trichloride to form a complex and then vulcanizing the complex (Japanese Patent Publication No. 2669/1956). In accordance with this method, equimolar amounts of aluminum trichloride and product PPTC are required. Furthermore, this method is complicated, especially as regards the treatment of aluminum chloride waste. Additionally, the method has significant industrial disadvantages due to the high cost of aluminum trichloride in the production of PPTC. It has also been known to produce PPTC by reacting benzene with phosphorus trichloride using thermal decomposition to form phenylphosphonous dichloride and then vulcanizing the product. In this method, since thermal decomposition at high temperature is required, a reactor made of anticorrosive material is necessary. Accordingly, it is a disadvantageous method.
On the other hand, it has also been disclosed to produce PPTC by reacting benzene with thiophosphoryl trichloride in an autoclave at 200.degree. - 450.degree. C in Japanese Patent Application Disclosure No. 86823/1973. In accordance with this method, the reaction is quite slow at about 200.degree. C and it has been necessary to heat the reactants to a temperature higher than 350.degree. C for effective reaction. Consequently, a reactor made of anticorrosive material having high compressive strength has been required for this process also. As a result, this method also has disadvantages for industrial operation. Finally, it has been disclosed that PPTC can be produced by refluxing benzene with thiophosphoryl trichloride in the presence of a Friedel-Crafts catalyst. In accordance with this method, a large amount of diphenylphosphonothioic monochloride by-product has been produced and about 12 - 15 hours of reaction time has been required. Accordingly, it also possesses significant industrial disadvantages. Consequently, it would be most desirable to have a process for preparation of PPTC which is very attractive industrially.